The present invention relates to a method for the detection of the rotational position of the rotor of a DC motor with commutator by evaluating the time development of a rotor current signal of the DC motor, which signal comprises a periodic waviness around an average value, and current ripples generated by commutation which are superimposed to said waviness. Such a method can be applied e.g. in actuating systems in order to detect, without using a sensor, the present moving position of an actuator driven by the DC motor. The method is particularly useful in automobiles, notably for automatic window lifts, electrically adjustable seats, a throttle valve in the internal combustion engine, as well as mixing valves and air-distribution valves, respectively, of the heating or air-conditioning system of the automobile.
It is known that the time development of the rotor current of a DC motor with commutator includes current peaks (hereinafter called current ripples) which can be attributed to a brief short-circuiting between two collector lamellae by the brushes of the DC motor. From DE 40 17 779 A1, DE 42 17 265 C2, DE 42 22 914 A1, DE 195 11 307 C1, EP 0 424 831 B1 and EP 0 459 435 B1, it is known that these current ripples can be counted in order to detect the rotational position of the rotor of the DC motor or the moving position of an actuator driven by the DC motor.
Further, it is known to examine the rotor current signal of a commutator DC motor on the basis of the waviness so as to be able to conclude to the rotational position or the number of rotations of the rotor. Thus, for instance, U.S. Pat. No. 5,898,288 describes the detection of the waviness of a rotor current signal but does exactly not describe the detection of current ripples.
In the above printed publication, reference is made to current ripples (see e.g. column 5, lines 30 to 33); however, what is illustrated in the drawings of said publication and what is really meant, after all, is the evaluation of the waviness of the rotor current signal. The waviness of the rotor current signal, however, is only one characteristic of a DC motor with commutator. Thus, superimposed to the wave-shaped rotor current signal are the current ripples which are formed as short “current peaks”. Exactly these current ripples, however, are not shown in U.S. Pat. No. 5,898,288. The fact that the waviness of the rotor current signal of a DC motor with commutator usually is wrongly equated with the “ripples” of the motor current, is further evident from DE-C-197 29 238. In this context, reference is to be made e.g. to the last sentence of the abstract of the cover sheet of this printed publication. The publication itself deals with the calculation of the point of time when a ripple (a half-wave) is to be expected in the rotor current signal. A half-wave, however, is clearly not a current ripple.
In known ripple count methods, a problem resides in the existence of load conditions and idle conditions of the DC current motor during which the detection of the current ripple is not performed with sufficient reliability. Further, depending on the respective use of a DC motor with commutator (DC motor with asymmetrical or symmetrical motor), it may happen that individual ripples are generated which, if they were included in the counting, would cause a faulty result.
It is an object of the invention to provide a method for the detection of the rotational position of the rotor of a DC motor with commutator by evaluating the time development of the rotor current signal of the DC motor, by which method the rotational position of the rotor and optionally, on the basis thereof, the current setting position of an actuator driven by the DC motor can be determined with still higher precision and reliability.